The present invention relates to a mixing device for manufacturing an aqueous fuel, and more particularly to a specially designed mixing device that creates a superior aqueous fuel emulsion from a hydrocarbon fuel, water, and an aqueous fuel emulsifier package.
Recent fuel developments have resulted in a number of aqueous fuel emulsions comprised essentially of a carbon-based fuel, water, and various additives, such as lubricants, emulsifiers, surfactants, corrosion inhibitors, cetane improvers, and the like. These aqueous fuel emulsions may play a key role in finding a cost-effective way for internal combustion engines including, but not limited to, compression ignition engines (i.e., diesel engines) to achieve the reduction in emissions below the mandated levels without significant modifications to the engines, fuel systems, or existing fuel delivery infrastructure.
Advantageously, aqueous fuel emulsions tend to reduce or inhibit the formation of nitrogen oxides (NOx) and particulates (i.e., combination of soot and hydrocarbons) by altering the way the fuel is burned in the engine. Specifically, the fuel emulsions are burned at lower temperatures than conventional fuels due to the presence of water. This, coupled with the realization that at higher peak combustion temperatures more NOx are typically produced in the engine exhaust, one can readily understand the advantage of using aqueous fuel emulsions.
As is well known in the art, the constituent parts of such aqueous fuel emulsions have a tendency to separate or be unstable over time because of the different densities or relative weights of the primary components, as well as other factors including the immiscibility of the compounds. As an example, middle distillate hydrocarbon sources have a density of about 0.85 while water sources have a density of about 1.0. Because the gravitational driving force for phase separation is more prominent for larger droplets of water, emulsions containing relatively smaller droplets of water will remain stable for longer periods of time. Aqueous fuel emulsion breakdown or phase separation is also influenced by how quickly the water droplets coalescence, or flocculate. The emulsion breakdown is also influenced by the environment in which the aqueous fuel is subjected. Any breakdown in the aqueous fuel emulsion can be extremely damaging if not detected before use in combustion. Given the microscopic nature of the suspended particles with the discontinuous phase, aqueous fuel emulsions can look acceptable to the naked eye but can actually be considered unacceptable when subjected to quality control standards to one skilled in the art.
Determining the amount of the emulsifier necessary for creating a specific emulsion of a water source and a hydrocarbon source can generally be calculated with calculations common to the art based on material densities, particle sizes of the discontinuous phase, etc. Such measurements are typically summarized in a particle distribution curve of the discontinuous phase.
It is commonly recognized that aqueous fuel emulsions can be produced by mixing a liquid hydrocarbon source, an emulsifier source, and a water source. The art of making aqueous fuel emulsions basically relates to three aspects:                1) The specific chemistries of the aqueous fuel emulsifier;        2) The specific sequences in which each of the ingredients (or portions thereof) are mixed with the other ingredients (or portions thereof); and        3) The specific mechanical mixing procedures of the ingredients.        
Chemistries for emulsifiers are generally composed of surfactants or soaps, among other things, that comprise a mixture of at least two components: one that is predominantly hydrocarbon soluble and the other that is predominantly water soluble so that the surfactant is balanced such that the interfacial tension between the hydrocarbon and water phases is substantially zero. In other words, each of these chemistries plays a critical role in breaking down the surface tension between the oil and water so a bond can form between the different molecules and to help disperse the water particles (from attracting to each other in the case of an oil phase). This is basically completed through three different types of electrical charged chemistries referred to as cationic (positive charge), anionic (negative charge) and non-ionic (neutral charge), or combinations thereof.
In many cases the emulsifier packages are designed to be soluble in the discontinuous phase. The amount of the emulsifier as a percent of the aqueous emulsified fuel will vary based on several factors which include the type and amount of continuous and discontinuous phase, the chemical composition of the emulsifier, and the particle sizes of the discontinuous phase.
While a range of different sequences have been recognized, it is generally understood that the principles of aqueous fuel emulsions dictate that the emulsifier supply should be mixed with the external phase of the aqueous fuel emulsion first (or portions thereof) and then with the discontinuous phase (or portions thereof) second.
For example, in the case of an oil-phased emulsion, the emulsifier supply would be first mixed with the hydrocarbon source before it is mixed with the discontinuous phase of water. Conversely, in a water-phased emulsion the emulsifier supply would be first mixed with the water source (or portions thereof) before it is mixed with the discontinuous phase of hydrocarbon fuel (or portions thereof). In the case where portions are premixed, the balance is introduced at a subsequent point as the aqueous fuel emulsion is manufactured.
While there can be several mixing stations during the emulsification process, a high-shear mixing stage is usually required when a water source is mixed with a hydrocarbon fuel source. Prior to the high-shear mixing, the various stages can be mixed with less intense mixing devices, such as in-line mixers or other common liquid agitators, because the chemicals being mixed have relatively compatible chemical properties. Because of the very different chemical properties of water and oil, significant amounts of mechanical energy are required to reduce the discontinuous phase to sizes where they can contribute to a stable aqueous fuel emulsion.
To date, high-shear mixers such as commercially available rotor-stator units and ultrasonic devices have been commonly referenced despite the fact that they were designed and sold primarily for the emulsification of non petroleum-related products such as foods products, cosmetic products and chemical products.
Several related art references have disclosed specific high shear devices for producing or blending a fuel emulsion. For example, U.S. Pat. No. 6,383,237 to Langer discloses the use of a rotor-stator mixer, when the hydrocarbon and water source are mixed, as does U.S. Pat. No. 5,873,916 to Cemenska. In both patents, the use of the commercially available high shear devices from well-recognized companies in the fluid agitation industry as part of their multi-step and multi-sequence fuel emulsion blending systems is disclosed.
Rotor stators basically provide shearing by a combination of a spinning blade, flow forced through a screen and/or a combination of both. Because the particle size of the discontinuous phase is largely determined by the shear rate of the high shear mixer, it is common for the discontinuous phase to have a wide range of particle sizes as a given portion is cut with the blade, a different portion is forced through a screen and another portion is subjected to both. To compensate for this occurrence many high shear mixers include dual or multiple staged rotor mixers or looped circuits, which allow aqueous fuel ingredients to be subjected to additional shear thereby increasing the population of uniform dispersed phase particle sizes. However, these additional high shear mixing devices or looped systems are more expensive and less efficient in terms of volume output, and are difficult to control correctly.
Despite the widespread use of high shear mixers in the aqueous fuel emulsion industry as well as other participants in the fluid agitation industry, there is almost no fundamental basis by which to theoretically predict or experimentally assess their performance. This fundamental is better illustrated through a general review of the shear rate and its calculation.
Shear is a force that is applied parallel to a surface, as illustrated in FIG. 1.
The forces are opposite as the square has to be in static equilibrium. This shear tends to elongate a solid, and in a liquid tends to create turbulence and eddies.
The shear formula that has been used for analysis of the physical processes in making emulsified fuels is as follows in FIG. 2 and Equation 1:
                              Shear          ⁢                                          ⁢          force                =                              V            ⁢                                                  ⁢            A            ⁢                                                  ⁢            u                    Bgc                                    Equation        ⁢                                  ⁢        1            
Where:                V is the velocity of the moving plate        A is the area of the plate        u is the viscosity of the fluid in question        gc is the gravitational constant, 32.2 ft/sec        B is the separation distance between plates.        
This equation was developed and is commonly used to determine the viscosity of liquids by measuring the force created by rotating a plate in the fluid of question. It is also directly applicable to any situation where one plate is moving in relation to another, such as in a colloid mill.
For flow between two surfaces, the physical situation is as follows in FIG. 3 and Equation 2:
                              Shear          ⁢                                          ⁢          force                =                              2            ⁢            V            ⁢                                                  ⁢            A            ⁢                                                  ⁢            u                    Bgc                                    Equation        ⁢                                  ⁢        2            
Where:                V is the velocity of the moving plate        A is the area of the plate        u is the viscosity of the fluid in question        gc is the gravitational constant, 32.2 ft/sec        B is the separation distance between plates.        
Although the linear velocity profile is an approximation (it is known that the velocity profile is parabolic in nature) it does provide a method for comparative calculation. As shear is present on both plates, the total shear force exerted on the fluid is about two times that from Equation 1.
One needs to recognize the fact that these calculations are not precise, as there are assumptions in their creation and in their application. However, these calculations illustrate the basic forces in fluid shear and can be used to develop relative force values for different shear modes.
Due to the rather imprecise methods available to calculate shear in commercially available unit scale-up and operation of these high-shear units as a component of a blending process of the aqueous fuel emulsions is generally completed by trial and error. Consequently, many of the commercial blending units available for blending aqueous fuel emulsions are configured around the limitations of the commercially available high shear units. This is one of the reasons the commercial aqueous fuel emulsion blending units require recirculation capabilities or multi-staged shearing (despite their higher costs or impact on lower capacity) to enable the water particles to be reduced to the desired particle size.
Because of problems inherent with the commercially available high shear mixing units such as the rotor stator, the effectiveness of the shear mixing units can only be varied by controlling the rate and frequency in which the emulsion material is subjected to high shear mixing. As stated above, the commercially available units may not be capable of creating a consistently uniform family of particle sizes of the discontinuous phase in the most practical and cost effective manner. This can create a fairly wide distribution curve for a family of particle sizes of water and in most cases creates a bi-modal curve. Having a consistent discontinuous phase particle size is not only important to create the foundation for a stable emulsion but it is critical in determining the required amount of emulsifier that is required. Consequently, it would be desirous to have a mixing system that creates a more uniform population of particle sizes of the discontinuous phase. A narrower particle distribution curve thereby creates an even distribution of the emulsifier sources between hydrocarbon source and the water source.